Polyurethanes are formed by the reaction of a polyisocyanate compound, such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) with a polyhydroxyl compound, such as a polyol. Generally, streams of approximately equal volume of the polyol and polyisocyanate are intermixed in a mixing head and then dispensed into a cavity or mold. In the mold or cavity, the hydroxyls of the polyol and isocyanates of the polyisocyanate react to form the polyurethane, while simultaneously water added to the polyol reacts with the isocyanates to provide CO.sub.2 in situ that causes the polyurethane to foam. Since isocyanates readily react with water, they are generally known to be an irritant and can be toxic when inhaled.
Prepolymers have been used in place of polyisocyanates to decrease the amount of vaporous isocyanate when making polyurethane foams. Prepolymers generally are made by reacting a diol with a diisocyanate monomer (e.g., MDI or TDI) at a great excess of NCO (i.e., isocyanate) to OH (i.e., hydroxyl). Commonly, the NCO to OH equivalence ratio is well in excess of 5. The ratio is large to keep the prepolymer from gelling while still providing enough NCO to produce a foam. As a consequence of the large NCO to OH ratio, a significant amount of diisocyanate monomer (e.g., MDI), typically, is still present in the prepolymer.
Generally, when making foams from these prepolymers, a gas blowing agent, such as a chlorofluorocarbon, is used to froth the prepolymer and ambient moisture reacts with the prepolymer to form polyurethane. Since ambient moisture causes the urethane reaction to proceed (i.e., formation of urea linkages and amines), foams of this sort suffer from inconsistencies depending on ambient humidity. Thick foams made by this method also tend to collapse as they cure. The foam collapses because an insufficient amount of water penetrates through the outer portion of the foam into the interior of the mass to adequately cure (i.e., cross-link) the interior of the foam.
More recently, International Patent Application No. 96/06124 has described prepolymers that have attempted to decrease the amount of diisocyanate monomer when making a foam. The 96/06124 application describes forming a prepolymer by reacting a polyisocyanate having an isocyanate functionality of 2.3 to 3.7 and having a diisocyanate monomer content of less than 20 percent by weight of the polyisocyanate with a polyol at high NCO to OH ratios (e.g., 4.6). The 96/06124 application also describes, when making a foam from these prepolymers, a blowing agent, such as a dimethylether, is used. Thus, this reference describes a technique of making a prepolymer with a low diisocyanate monomer concentration by starting with a polyisocyanate having a low concentration of diisocyanate monomer initially. This technique suffers from the need to use a polyisocyanate that must have a low diisocyanate monomer content initially, which may require, for example, expensive distillation or reaction processes to form said polyisocyanate. These prepolymers also require a gaseous blowing agent (e.g., CFCs) when making a foam.
Thus, it would be desirable to provide a prepolymer composition, method to form the prepolymer composition, foam made from the prepolymer composition and method to make the foam that avoids one or more of deficiencies of the prior art, such as one of those described above.